1. Field of the Invention
The present invention relates to silicone rubber compositions and to long-term-stress-resistant silicone elastomers obtained therefrom which exhibit excellent resistance to long-term dynamic mechanical stresses, and to their use.
2. Background Art
Silicone elastomers are frequently used in applications which are associated with high dynamic mechanical stresses. Examples are oscillation dampers, resonance dampers, exhaust and catalyst suspension elements, and engine mounts in automobiles. Further examples include mountings for movable parts in machine construction, peristaltic pump tubing in the medical field, diaphragms, valves, and in particular, keypads commonly used in the electronics field. To maintain the function of these silicone elastomer components, it is particularly important that the mechanoelastic properties either exhibit no change or change only slightly when dynamic mechanical stress is applied repetitively. However, repeated dynamic mechanical stress cycles generally lead unavoidably to continually increasing mechanical fatigue and damage to all elastomer materials, and this ultimately leads to a loss of function of the elastomer component. The keypads made of conventional silicone elastomers can, for example, withstand several million deformation cycles until their function is lost completely.
To meet the increased requirements in terms of the service time of elastomer components subjected to dynamic mechanical stress, numerous methods of increasing the resistance to dynamic mechanical stress, known as long-term stress resistance or fatigue resistance, durability or flex life, have been developed.
U.S. Pat. No. 4,745,144 describes silicone compositions to which particular organophosphorus compounds have been added for the purpose of increasing the long-term stress resistance, as a result of which a long-term stress resistance determined by the de Mattia test in accordance with JIS K-6301 of, for example, about 7 million deformation cycles can be achieved. However, the presence of organophosphorus compounds in silicone compositions or silicone elastomers has a number of disadvantages. For example, platinum-catalyzed addition crosslinking is inhibited, compression set is higher, thermal stability is reduced, and undesirable, and at times, toxic decomposition products are formed upon heating.
EP 0 841 363 A1 discloses a method for preparing silicone rubber compositions continuously. Due to the use of specific siloxanols which promote wetting of the filler, they addition crosslink to form silicone elastomers having an increased long-term stress resistance. The long-term stress resistances determined in accordance with JIS K-6301 by the de Mattia test reach values of up to 14 million deformation cycles. However, a disadvantage is that the preparation of the required siloxanols, which proceeds via a palladium-catalyzed hydrolysis step, is relatively complicated. In addition, stabilizing additives are necessary for storage of such siloxanols and the silanol groups which have not reacted in the silicone composition can lead to a viscosity increase during storage, known as “crepe hardening”.
EP 0 501 380 A2 discloses that the mechanical long-term stress resistance of silicone elastomers can be considerably improved by addition of a siloxane which has a carbinol group bound via a divalent hydrocarbon radical to Si. Long-term stress resistances of, for example, about 4 million stress cycles were able to be achieved in the de Mattia test, in accordance with JIS K-6301, by this method. A disadvantage is the relatively complicated preparation of carbinol-functional siloxanes. In addition, such compounds have a thixotropic effect, i.e. the flowability of the silicone composition is considerably impaired. Furthermore, the relatively long-chain alkylene spacers in the carbinol-functional siloxane also lead to a reduction in the thermal stability.
EP 0 497 277 A2 describes crosslinkable silicone compositions which contain an additional constituent in order to increase the resistance of silicone elastomers to repetitive deformation. The additional constituent is an organooligosiloxane which has from 2 to 10 Si atoms per molecule, at least one Si-bonded alkenyl group, and at the same time, at least two Si-bonded hydrolyzable groups. In this way, long-term stress resistances of about 4 million stress cycles are achieved in the de Mattia test. However, additives having hydrolyzable groups can cause a massive increase in viscosity in flowable silicone compositions, which is why this method is only suitable for solid silicone rubber compositions.
All approaches according to the prior art for increasing long-term stress resistance are based on addition of specific additives. However, this results in many disadvantages in terms of the performance of the silicone elastomers, for example reduced thermal stability and an increase in viscosity. In addition, the long-term stress resistances obtainable by means of these methods no longer meet increased requirements with respect to the service time of silicone elastomer components subjected to dynamic mechanical stress.
One starting point for a solution is based on the fact that low filler content and high filler dispersibility have an advantageous effect on long-term stress resistance. Furthermore, service time of elastomer components subjected to dynamic mechanical stress increases with increasing mechanical strength, for example tear propagation resistance or ultimate tensile strength of the elastomer. As described in G. J. Lake, A. G. Thomas, PROC. R. SOC. LONDON SER. A, A300, 108-119 (1967), the energy required for propagation of a crack is proportional to the square root of the mean molecular weight of the polymer chain located between two nodes of the network. Thus, the long-term stress resistance of, inter alia, a silicone elastomer should be able to be increased by setting a low crosslink density and thus a relatively low modulus. In agreement with this, DE 30 12 777 A1 discloses that the flex life of a peristaltic pump can be increased by selection of a silicone elastomer having a relatively low modulus.
EP 0 798 342 A2 describes addition-crosslinkable silicone rubber compositions which can be used to produce keypads which display improved hysteresis behavior, i.e. the actuated keypad returns to its original position more quickly and more completely. Associated therewith is a more pronounced response of the keypad which the user of the keypad perceives as stronger snapping-back and as a cracking which confirms depression, referred to as crispness. The improved hysteresis behavior is achieved by a combination of a polysiloxane having vinyl functionality exclusively at the ends of the chain, i.e. an α,ω-vinyl-functional polysiloxane, with a polysiloxane which additionally contains from 1 to 5 mol % of vinyl groups in the chain, and a reinforcing silica which has been treated with a mixture of tetramethyldivinyldisilazane and hexamethyldisilazane. Direct comparison with the approaches described above is not possible, since no information about the long-term stress resistance is given. With regard to the material composition, similar silicone rubber compositions are described in EP 0 305 073 A2. Here too, improved values for the ultimate tensile strength and tear propagation resistance are achieved by means of a combination of a polysiloxane having vinyl functions only at the ends of the chain with a polysiloxane which additionally has from 1 to 5 mol % of vinyl groups in the chain.
The hysteresis behavior described in the patent application EP 0 798 342 A2 relates to the mechanical recovery behavior of the elastomer, i.e. to the percentage decrease in the height of the specimen observed immediately after actuation, i.e. pressing down and letting go, of a keypad-like specimen. The mechanically incomplete or delayed recovery to the initial state indicates viscous, dissipative processes. Quite generally, all viscoelastic materials display a more or less pronounced hysteresis behavior on going through dynamic mechanical stress cycles, i.e. the stress-strain curve of a deformation cycle encloses an area which corresponds to the deformation energy dissipated per unit volume during the deformation cycle, known as the specific energy loss. The specific energy loss is consequently a characteristic measure of the internal friction occurring in a material when it is subjected to dynamic-mechanical stress.
In view of the above-described prior art, it is to be expected that low-modulus silicone elastomers having a low specific energy loss will have the highest long-term stress resistances. However, a direct comparison of the second approach with the first approach cannot be made, since no determination of the long-term stress resistance by means of the de Mattia test in accordance with JIS K-6301 was carried out.